Milling machines can be programmed so their spindle and mill holding collet move in a desired path. However, milling machines which employ end mills are subjected to side cutting loads which result in cantilever or side deflection of the end mill away from the cutting surface, with the tip of the mill deflecting more than the shank. As a result, both the side-milled surface and the end-milled surface are not true to the part and deviate from the prescribed path.
In critical applications, such as the milling of scroll parts for scroll gas compressors, in which the vertical walls and bottom surfaces of the scroll parts form the sealed gas pockets, any error in the machining of the parts prevents precise seal contact. The resulting gas leak from a pocket of high pressure to a pocket of lower pressure causes loss of compressor efficiency.
Several factors affect the slant of the walls, among which are:
(1) End mill characteristics including tool grind, tool dullness, modules of elasticity of tool material, mill flute depth, mill diameter and amount of extension of the mill beyond the mill collet; PA1 (2) Rigidity of the mill collet; PA1 (3) Mill spindle characteristics including shaft size, bearing size, bearing distance behind the end mill and the bearing preload; PA1 (4) Depth of cut, feed rate, spindle RPM and material hardness; PA1 (5) The number of flutes cutting at one time. In the middle of the scroll, multiple flutes may be cutting simultaneously due to the part wall wrapping around the end mill. At the outer extremity of the scroll wall, only one flute may be cutting at a time due to the reduced wrap of part around the end mill; PA1 (6) The rigidity of the wall, which in the middle of the scroll, is relatively stiff due to the small curvature of the wall, but which at the scroll outer extremity is much more compliant due to the wall approaching flatness and the lack of support at the end of the wall; and PA1 (7) Inner wall milling versus outer wall milling in which the wall wrap around the end mill is greatly reduced and the forces causing slant are reversed from inward slant to outward slant.
Attempts have been made to correct the wall slant by reverse taper grinding the cylindrical portion of the end mill flutes. The nature of the factors mentioned above renders it impractical if not impossible to predict the exact amount of reverse taper required to make the correction. Also, since the slant effect varies as the mill cut progresses from the inner circle to the outward extremity of the scroll, the end mill taper grind requirements vary during the cut. Also, taper grinding does nothing to correct bottom end mill cut.
To best establish the necessary slant corrections, and in accordance with the present invention, a sample part can be milled under real world or actual working conditions in which all of the above error factors are in effect. Then the part is inspected for slant at various increments throughout the milling of the inside and outside scroll wall. The inspection results are loaded into the tool offset tables of a computer numerical control (CNC) at the matching intervals of the CNC program.
This invention provides the means and the mechanism to utilize the above inspection information to correctly cut subsequent parts, with slant corrections changing throughout the milling cut program.
Preferably, the mill spindle is mounted in a cage for pivotal movement to correct its position as necessary. The cage is tilted by a tilt servomotor as controlled by the CNC program, with position monitored by an encoder. When additional parts are machined, the tilt servomotor continuously corrects the angular position of the mill to compensate for the varying end mill deflection found in the sample cut part.
Preferably, the pivotal mounting for the mill spindle cage is provided by a pair of flexure trunnions mounted on opposite sides of the centerline of the end mill and spindle. Since the end mill receives cantilevered support from a holding collet and the bending of the end mill is concentrated a short distance beyond the collet face, the pivotal mounting should be located at a level near the face of the collet.
One object of this invention is to provide means for compensating for side deflection of an end mill having the foregoing characteristics and capabilities.
Another object is to provide side deflection compensation apparatus for an end mill which is constructed of a relatively few simple parts, is rugged and durable in use, and is relatively inexpensive to manufacture, assemble and operate.
These and other objects, features and advantages of the invention will become more apparent as the following description proceeds, especially when considered with the accompanying drawings.